1. Field of the Invention
The present invention relates to a thermal head having a bent electrode structure and a method of manufacturing the same.
2. Description of the Related Art
A known thermal head usually comprises a bent electrode structure, a common wiring line and an individual wiring line. The bent electrode structure includes where a bent electrode, which serves to electrically connect a pair of adjacent heating resistors to each other. The common wiring line and an individual wiring line are used to cause the pair of heating resistors to be electrically conducted through each of the bent wiring lines. The electrode structure causes a plurality of heating resistors, which are arranged at predetermined pitches there between on a glazed substrate, to electrically conduct. Electrode wiring lines in the bent electrode structure are becoming finer as the density increases and substrates become smaller. Particularly in the case of a common wiring line located between bonding pads of individual wiring lines, a line and a corresponding space (electrode width and a gap between electrodes) are very narrow, that is, currently about 12 μm and 6 μm, respectively, since it is difficult to make the bonding pads for driver ICs formed at one ends of the individual wiring lines small more than a predetermined amount.
When a protruding level difference glazed substrate having a protruding level difference part is used as a glazed substrate, the electrode wiring lines (bent electrodes, common wiring line, and individual wiring lines) are obtained by forming a resistor layer on the entire surface of the protruding level difference glazed substrate, forming the resistor layer as a resistor pattern having a predetermined shape using a photolithographic method, and then forming a conductor layer on the resistor pattern excluding a region where a plurality of heating resistors on the protruding level difference part are formed. Specifically, when a photolithographic process is used for forming a resistor pattern, a resist is coated on a resistor layer, the resist is exposed and developed to form a resist pattern for obtaining a predetermined resistor shape. The resistor layer is etched using the resist pattern as a mask, and the resist is removed. When forming the resist pattern, the entire resist is exposed in a state in which an exposure focus is focused on a top portion of the protruding level difference part in order to define the planar size of the heating resistors with high accuracy (one-shot exposure: see FIG. 5). The conductor layer is formed using the photolithographic method (resist coating, exposure, development, etching, resist removal) in which some resistor patterns formed at top and bottom portions of the protruding level difference part are used as upper and lower alignment marks (positioning indicators). The resistor pattern located directly below the conductor layer serves as an adhesive layer for improving the adhesion between the conductor layer and the glazed substrate.
In recent years, as the density increases and a substrate become smaller, a level difference of the protruding level difference part of the protruding level difference glazed substrate tends to increase. For this reason, when forming the electrode wiring lines, an exposure focus at the bottom portion of the protruding level difference part is not clear in a one-shot exposure (in which the entire resist is exposed in a state where an exposure focus is focused on the top portion of the protruding level difference part). As a result, the patterning accuracy is lowered. It is desirable to secure sufficient line and space for electrode wiring lines. However, it has been difficult to form a fine electrode wiring line in a narrow wiring region where a fine electrode wiring line, such as the common wiring line located between the bonding pads, is needed. For example, the electrodes have been short-circuited because the conductor layer or the resistor layer between electrodes is not completely removed. As a measure to solve this problem, a two-step exposure has been considered. Here an exposure focus is focused on each of the top and bottom portions of the protruding level difference part (see FIG. 6) and use a resist pattern of the top portion and a resist pattern of the bottom portion to thereby improve the patterning accuracy.
However, in order to form a conductor layer using the two-step exposure, alignment masks provided at the top and bottom portions respectively of the protruding level difference part are used for alignment with the resistor pattern. The alignment marks need to be simultaneously formed at the top and bottom portions of the protruding level difference part in the same process using a part of the resistor pattern. However, when forming the resistor pattern using the two-step exposure, the alignment marks of the top and bottom portions cannot be formed at the same time. As a result, positional deviation occurs in the alignment marks. Accordingly, in the case where the conductor layer is formed by using the alignment marks as indicators, pattern deviation at the top and bottom portions of the protruding level difference part and pattern deviation with respect to the resistor pattern become too large. This also makes it difficult to obtain fine electrode wiring lines.